1. Field of the Invention
This invention relates generally to a miniature motor, and more particularly to a miniature motor having a positive-coefficient thermistor in which the overheat of the miniature motor is prevented by effectively detecting motor temperature to control overcurrent by the positive-coefficient thermistor mounted on a motor cover plate, and the size of the motor is reduced and productivity is improved without adding to the number of parts or increasing the size of the motor by providing the positive-coefficient thermistor.
2. Description of the Prior Art
FIG. 1 is a diagram illustrating a conventional type of miniature motor which is publicly known and has essentially the same construction as the present invention. The miniature motor shown in FIG. 1 comprises terminals 11 supported by a motor cover plate 10, brushes 12 making contact with a commutator 4 fixedly fitted to a motor shaft 14; and electric current is fed to rotor windings 5 wound on a rotor core 15 fixedly fitted to the motor shaft 14 via the commutator 4, and thereby a rotor 18 existing in a field formed by a parmanent magnet 17 fixedly fitted to the inner circumferential surface of a motor case 16 is caused to rotate.
The miniature motor shown in FIG. 1 has wide applications, including automotive electrical components (such as motor-driven rear-view mirrors, motor-driven windows, etc.), motor-powered toys, and tape recorders. Having a low output, the miniature motor tends to be overloaded due to a trifling trouble in the driven parts (such as rusting, entry of dirt and dust, for example), leading to the overheat or burnout of the rotor windinigs.
The most commonly used method of preventing the rotor windings from being overheated is to cut off current in the event of overload, using a bimetal, fuse, or heat-sensitive element relay.
However, these conventional methods have various problems: (1) The bimetal method is poor in operating performance at low voltages; (2) the fuse method involves the changing of fuses when burned out; (3) the heat-sensitive element relay method results in the increased size of the circuit device.
As an overheat-preventing means that can solve these problems there is a well-known method of feeding power to a miniature motor M from a power source 21 via a positive-coefficient thermistor 19, as shown in FIG. 2. The positive-coefficient thermistor 19 is usually disposed in such a manner as to come in close contact with the inside of the miniature motor M or the surface of the motor case in order to effectively detect the temperature of the miniature motor M.
The positive-coefficient thermistor 19 has such a characteristic that its resistance sharply increases as temperature exceeds a certain level (100.degree. C., for example). Consequently, if a sustained overload applied to the miniature motor M, or a forcible locking of motor revolution, would produce overcurrent in the rotor winding 5, the temperature of the miniature motor M rises, causing the internal resistance of the positive-coefficient thermistor 19 to rapidly increase, leading to a sharp decline in the current fed to the miniature motor M and thereby successfully preventing overheating of the miniature motor M.
In the conventional type of miniature motor having the aforementioned positive-coefficient thermistor, the positive-coefficient thermistor is usually fitted to the outside or the inside of the motor in such a manner as to make close contact with the motor case.
A miniature motor having a positive-coefficient thermistor outside the motor requires a separate housing device, equipped with connecting terminals, etc., for housing the positive-coefficient thermistor. This results in an increased number of parts involved as well as in the increased manhours and cost of assembly. Installation of the thermistor outside the motor also poses an unwanted problem of a protruded profile of the motor.
A miniature motor having a positive-coefficient thermistor inside the motor, on the other hand, involves a space for housing the positive-coefficient thermistor. To cope with this, a motor-driven device having an overheat preventing function has been proposed in Japanese Laid-Open Utility Model Application No. 120226/1986, in which a motor case is bulged to provide a space for housing the positive-coefficient thermistor. With this construction, however, the fabrication of the motor case requires much labor, offering an unwanted structural problem of an additional projection.